Submission #2140482

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;

using s64 = long long;

const int MaxN = 500000;
const int MaxM = 500000;
const int M = 998244353;

inline int modpow(int a, const int &n)
{
	int res = 1;
	for (int i = n; i; i >>= 1)
	{
		if (i & 1)
			res = (s64)res * a % M;
		a = (s64)a * a % M;
	}
	return res;
}

int n, m;

int fact[MaxM + 1];
int rfact[MaxM + 1];

inline int binom(int n, int m)
{
	return m < 0 ? 0 : (s64)fact[n] * rfact[m] % M * rfact[n - m] % M;
}

int main()
{
	cin >> n >> m;

	fact[0] = 1;
	for (int i = 1; i <= m; ++i)
		fact[i] = (s64)fact[i - 1] * i % M;
	rfact[m] = modpow(fact[m], M - 2);
	for (int i = m; i; --i)
		rfact[i - 1] = (s64)rfact[i] * i % M;

	int res = 0;
	for (int i = max(n, (m + 1) / 2); i <= m; ++i)
	{
		int j = m - i - (i * 2 == m);
		int t = i - n + 1;
		res = (res + binom(i + j, j) - binom(i + j, j - t)) % M;
	}

	cout << (res + M) % M << endl;

	return 0;
}

Submission Info

Judge Result